As of late 2015, Apple has changed the technology type in the 27" 5K Retina and 21.5" 4K Retina iMacs to an RG Phosphor backlight. As a result, the appropriate technology type to select with the ColorMunki Display is "RG Phosphor" and "GB-LED" (same calibration matrix as RG Phosphor) with the i1Display Pro. ColorMunki Smile users will need to stay with "LED". While Apple went to white LED backlit displays in 2009, these two iMac models switched to the RG Phosphor in late 2015. Prior to 2009, iMacs were CCFL backlit displays. To properly identify an RG Phosphor iMac, go to the apple in the menu bar and select "About this Mac". If the resulting window shows that the iMac is "Late 2015" or newer, it is the RG Phosphor / GB-LED technology type. To summarize the history of iMac Technology Type: